The invention described herein may be used and/or licensed by or for the Government of the United States of America.
1. Field of the Invention
This invention relates generally to molecular beam epitaxy (MBE) systems and methods of operation.
2. Discussion of The Related Art
MBE machines are mostly started using a mix of computer program tools and manual decisions and actions. The goal of the start up process is to warm up a certain number of evaporation cells from an idling temperature to a higher operating temperature at which they produce flux levels that in turn produce a certain growth rate of a crystal when allowed to impinge on a substrate. Before such a warm-up is started, it is highly desirable to know the pressure in the MBE vacuum system so that the process can be aborted if the pressure is too high.
The target fluxes are usually calculated using proprietary software written in an arbitrary computer language or set up as a spreadsheet. The algorithm in such software is trivial and commonly known. It is assumed that the logarithm of the beam flux is linearly dependent on the inverse of the absolute temperature of the cell times the square root of the temperature. The linear relationship is determined occasionally in situ by beam flux measurements at a set of temperatures. Furthermore, it is assumed that the growth rate is directly proportional to the flux. The latter proportionality is determined experimentally, either using reflection high-energy electron diffraction (RHEED) oscillations or by growth of one or more test structures using carefully measured fluxes at specific temperatures and for specific times with subsequent ex-situ measurement of thickness and/or compositions to determine the resulting growth rates.
Dopant fluxes are too small to be measured directly using a beam gauge. They are therefore usually calibrated by growth of one sample with several layers using different doping cell temperatures, or by growth of several samples at different temperatures. In the first case the sample can be evaluated using electro-chemical capacitance-voltage profiling and in the second case also by Hall effect measurements. Once measured, a linear relationship between the logarithm of the doping concentration and the inverse absolute temperature is determined and normalized to a specific growth rate of the matrix material. Using this relationship, the dopant cell temperature that produces an arbitrary dopant level at an arbitrary growth rate can be calculated.
Commonly the operating temperatures calculated in this way are entered in a control computer, which can ramp the cell set points from an idling level to the desired targets. This process takes at least xc2xd hr and is preferably done before work hours. In other words, the computer program is started during the previous workday and set up to start a certain period of time before the operator is expected to arrive at work. However, if no provision is made to check the vacuum integrity of the system before this process is started, a certain risk exists that the ramping will damage the system. Some operators write custom software to perform this check while others simply chose to accept the risk.
Once the cell temperatures have reached their target values (typically after a brief overshoot to provide cleaning) the fluxes must be measured again. The reason is that some hysteresis may be characteristic of the behavior of the cells and that evaporants have been consumed in various amounts since the last measurement. Thus, the calculated temperature will usually not produce the desired flux with acceptable accuracy. The temperatures must then be adjusted iteratively based on measured values. This process can be very time consuming and must follow a rigid protocol to ensure that reproducible numbers are obtained.